In fixed-wing aircraft, ground effect is generated by an aircraft's wings when they are close to a solid, fixed surface that results in increased lift and decreased drag and which requires very little thrust (forward horsepower). Ground effect increases air pressure on the lower wing creating a “ram” or “cushion” effect which greatly improves the lift to drag ratios by up to 250%. By reducing the drag coefficients, the thrust or energy expended to maintain speed is also greatly reduced. Every aircraft from jumbo jet airliners to small Piper Cubs, experience ground effect, which is especially noticeable when landing, as the aircraft momentarily “floats” above the runway. When taking off, ground effect may temporarily reduce the stall speed. The pilot can then fly level just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached.
In addition to traditional aircraft, wing-in-ground (WIG) watercraft that use ground effect to fly above water are also known in the art. WIG craft are used primarily over water due to the relatively constant surface of water that is free of obstacles. Generally, such watercraft have large fixed wings, about 1½ times greater than the height of ground effect in which they fly, the ground effect extending approximately 10-30 feet above the surface of the water. WIG craft largely travel at very high speeds, above approximately 50 mph, and as high as 100 mph or greater, which is achieved by using small engines. They also include a fuselage or hull that travels in the water when not in ground effect. WIG watercraft are desirable, particularly as transport vehicles, because they are more fuel-efficient than conventional watercraft, utilize small engines and are capable of travel at high speeds which can reach over 100 mph, thus covering large distances quickly.
A hydrofoil is a lifting surface, or foil, that generally operates in water. Hydrofoils are similar in appearance and purpose to airfoils, which are used by airplanes. As a watercraft using hydrofoils gains speed, low pressure is developed above the foil and high pressure is developed below the foil creating lift. When used as a lifting element on a hydrofoil craft, this upward force lifts the body or hull of the craft, decreasing drag and increasing speed.
There are two basic types of hydrofoils; “surface piercing” where the foil comes out of the craft and enters the water, usually at an angle so as the craft lifts there is less foil in the water, thus reducing drag as speed increases; and “submerged foils” that are completely underwater either fixed or dropped over the side of the craft. Submerged foils are not self-stabilizing, so they are continuously tilted to gain lift from the angle, similar to tilting an airplane nose up or down to change the angle of attack of the wings.
With either submerged or piercing foils if the sea state or waves are higher than the depth of the foils, then the bottom of the craft crashes into the waves, causing the craft to slow down and the foil lift speeds to decrease. A high speed hydrofoil shape must always have water flowing over and under the foil to work or create lift. Another problem associated with either submerged or piercing foils hydrofoils is “sonic cavitation”. Because water is 700 times more molecular dense than air, once a hydrofoil reaches speeds much over 60 mph molecular “bubbles” from the top and bottom of the foil crashing into each other at the trailing edge of the foil causing cavitation. Similar to a separation bubbles in air, cavitation largely increases drag and often also reduces lift, thus resulting in loss of speed. Additionally, the collapse of larger vapor bubbles has been found to lead to vibrations and even structural damage. Damage due to cavitation often is a problem for marine propellers, turbines and pumps.
As a result of these shortcomings, it appears that the speed “wall” for hydrofoils is around 60-70 mph and conventional hydrofoils generally cannot handle seas above about 6-12 feet. Thus, hydrofoils have experienced very limited commercial application and success.
First developed in 1923, gyrocopters or gyroplanes are wingless aircraft, similar in look to a helicopter, which use auto gyration, i.e. free spinning, non-powered rotors, to obtain lift. Auto gyration occurs when air is passed under a rotor blade causing the blades to spin which then provides lift, while an engine turning a conventional airplane propeller creates forward motion or thrust. Pitch control is achieved by tilting the rotor fore and aft; roll control by tilting the rotor laterally (side to side). A gyrocopter cannot lift straight up or hover like a helicopter, and requires a relatively short runway for takeoff and landing. One reason that gyrocopters did not gain mass appeal is their lack of speed. The physics that create auto gyration also impedes speeds much above 120 knots due to the lift/drag ratios of the rotors. By 1939, the gyrocopter concept was largely discarded because aircraft manufacturers for military and civilian use were hoping to achieve speeds well in excess of 300 knots for propeller driven aircraft.